1. Field of the Invention
The present invention relates to a light-emitting element using electroluminescence (EL). Further, the present invention relates to an organic compound included in the light-emitting element. Furthermore, the present invention relates to a light-emitting device, an electronic device, and a lighting device each of which includes the light-emitting element.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements utilizing EL. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Such light-emitting elements are self-luminous elements and have advantages over liquid crystal displays in having high pixel visibility and eliminating the need for backlights, for example; thus, such light-emitting elements are thought to be suitable for flat panel display elements. The light-emitting elements also have a great advantage that they can be manufactured as thin and lightweight elements. Further, very high-speed response is also one of the features of such elements.
Further, since such a light-emitting element can be formed in a film form, planar light emission can be easily obtained. Therefore, large-area elements utilizing planar light emission can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source applicable to a lighting device and the like.
The light-emitting elements utilizing EL can be broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as the light-emitting substance, application of a voltage to a light-emitting element causes injection of electrons and holes into a layer that includes the organic compound from a pair of electrodes, and thus a current flows. Then, the injection of these carriers (the electrons and holes) makes the organic compound to form an excited state and to emit light when it returns from the excited state to a ground state.
With such a mechanism, such a light-emitting element is called a current-excitation light-emitting element. Note that the excited state formed by an organic compound can be a singlet excited state or a triplet excited state, and luminescence from the singlet excited state is referred to as fluorescence, and luminescence from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are a lot of problems which depend on a substance, and in order to solve the problems, improvement of an element structure, development of a substance, and the like have been carried out (for example, see Non-Patent Document 1).